Dispersion strengthened nickel-chromium alloys

ABSTRACT

HIGH TEMPERATURE AND OXIDATION RESISTANT ALLOY PRODUCTS CONSISTING OF A NICKEL MATRIX CONTAINING UNIFORMLY DISSEMINATED SUB-MICRON SIZED REFRACTORY OXIDE PARTICLES AND HAVING DEPOSITED THEREON UP TO ABOUT 35%, BY WEIGHT CHROMIUM WITH THE HIGHEST CONCENTRATION OF CHROMIUM BEING AT THE SURFACE OF THE STRIP. THE MATRIX IS SUBSTANTIALLY FREE OF RECRYSTALLIZATION AND HAS ELONGATED, FIBROUS GRAINS WHICH HAVE A STABLE SUB-STRUCTURE.

Patented Sept. 19, 1972 U.S. Cl. 148-31.5 3 Claims ABSTRACT OF THE DISCLOSURE High temperature and oxidation resistant alloy products consisting of a nickel matrix containing uniformly disseminated sub-micron sized refractory oxide particles and having deposited thereon up to about 35%, by weight chromium with the highest concentration of chromium being at the surface of the strip. The matrix is substantially free of recrystallization and has elongated, fibrous grains which have a stable sub-structure.

This invention relates to a process for producing dispersion strengthened nickel base alloys containing between and 35% by Weight chromium and to the novel wrought dispersion strengthened alloy products obtained in accordance with the process.

This application is a divisional application Ser. No. 815,780 filed Apr. 14, 1969, and now abandoned, which in turn is a continuation-in-part of Ser. No. 570,389 filed July 22, 1966, now U.S. Pat. No. 3,454,431.

It is well known that the high temperature oxidation resistance and strength of nickel base alloys can be greatly improved by alloying with chromium. Improvement in high temperature strength characteristics is generally obtained with the addition of more than 10% by weight chromium and high temperature oxidation resistance is improved by the presence of at least by weight chromium. The upper limit of useful chromium content in nickel alloys is about 35 by weight, at which point the solubility limit of chromium and nickel is reached. It has long been conventional in the art to alloy nickel With about by weight chromium to obtain optimum improvement in oxidation resistance and strength.

It is also known that the high temperature senvice characteristics of metals and particularly of nickel and nickel base alloys can be substantially improved by dispersion strengthening. Dispersion strengthening involves the provision within a matrix metal of a large number of fixed, uniformly disseminated, sub-micron sized refractory particles. These dispersed particles, which are normally refractory oxide particles such as thoria, function to stabilize the matrix metal at elevated temperatures thereby increasing its tensile strength and stress-to-rupture life.

In addition, it is known that the micro-structure of a metal or metal alloy strongly influences its strength characteristics and, further, that such structure is determined to a large extent by the manner in which a material is fabricated and worked. For example, co-pending U.S. application Ser. No. 465,291, filed June 21, 1965 (now Pat. No. 3,366,515) describes one method whereby a novel, strength enhancing grain structure can be developed :in dispersion strengthened nickel and nickel base alloys. According to this method, a substantially fully dense billet comprised of matrix metal and highly disseminated submicron refractory oxide particles is subjected to a series of controlled work cycles in which the billet is cold worked to take reductions in cross-sectional area of less than 20%, preferably less than 10%, followed by a controlled anneal after each such reduction to relieve residual stresses within the workpiece. This working method pro duces a matrix micro-structure comprised of small, elongated fibrous or lamellar grains having a polygonized substructure is characterized by little evidence of recrystallization and relatively small, equiaxed sub-grains bounded by low angled grain boundaries. This particular microstructure substantially enhances the strength properties of the wrought, dispersion strengthened material in which it is developed.

Efforts to produce nickel base alloys which combine the desirable high temperature service characteristics separately obtainable from alloying nickel and chromium, dispersion strengthening and specific grain and sub-grain structure control encounter many difficulties. These result largely from problems inherent in the powder metallurgy techniques conventionally employed in the production of dispersion strengthened alloys. For one thing, metal powders as used for this purpose are particularly susceptible, because of their large surface area, to contamination when exposed to air. In air, thin coatings of oxides and nitrides are formed on the particle surfaces. Such coatings on nickel particles may be removed or cleaned up satisfactorily by heating the particles in a reducing atmosphere such as in hydrogen gas. The contaminant coatings associated with chromium particles, on the other hand, are notoriously difficult to clean up; heating in the presence of hydro gen has little or no affect.

Such contaminant coatings on the chromium particles hinder the inter-diffusion of chromium and nickel when conventional powder metallurgy methods involving powder blending compaction and sintering are employed to fabricate dispersion strengthened nickel-chromium alloys. As a result, only incomplete homogenization of the nickelchromium matrix is obtained and the benefit which can normally be derived from alloying 10% to 35 by weight chromium with nickel is not fully realized. In addition, contaminant coatings are apparently a factor in causing undesirable agglomeration of the refractory particles during processing which further adversely affects the high temperature service characteristics of the alloy product.

Another problem is that fully dense dispersion strengthened nickel-chromium alloys produced by conventional powder metallurgy procedures do not normally have a grain or sub-grain structure which contributes appreciably to their strength properties and known methods for developing a desirable micro-structure have been found ineffective or impractical when applied to nickel-chromium alloys containing a dispersed phase. A fibrous grain structure with polygonized sub-grain structure can be developed in the matrix of such alloys by utilization of the working method of United States application Ser. No. 465,291 (now Pat. No. 3,366,515) noted above, but it has been found that while this method is particularly suitable for fabrication of dispersion strengthened nickel products, it is generally uneconomic for fabricating dispersion strengthened nickel-chromium alloys on a commercial basis.

The present invention provides a practical and effective method for producing wrought dispersion strengthened alloy products with a nickel-chromium matrix with uniformly dispersed refractory particles and any predetermined characteristic matrix micro-structure which can be developed in dispersion strengthened nickel. Alloys obtained by the method of the invention possess the superior high temperature strength properties and oxidation resistance normally attributable to nickel-chromium alloys and have further enhanced high temperature strength characteristics attributable to the uniformity of distribucontrolled micro-structure.

The process of the invention derives from the discovery that under certain conditions substantially fully dense wrought dispersion strengthened nickel having any characteristic micro-structure can be alloyed with chromium by high temperature solid state infusion of the chromium into the nickel matrix without causing agglomeration of the dispersed particles of recrystallization of the matrix and While retaining in the nickel-chromium matrix of the final product the characteristic micro-structure of the original nickel matrix. Thus, the invention permits the alloying of chromium with dispersion strengthened nickel having the uniform dispersoid distribution and preferred micro-structure which can be readily developed in straight nickel-dispersoid wrought products but which heretofore could not be practically and economically achieved in nickel-chromium-dispersoid wrought products.

In a preferred embodiment of the invention, chromium is heat diffused into a wrought dispersion strengthened nickel shape having a characteristic microstructure developed by its previous processing history such that the shape has a relatively high chromium concentration at it surface and decreasing chromium concentration inwardly from the surface. Surprisingly, despite the extensive heating and the massive infusion of chromium into the nickel matrix with corresponding increase in the volume thereof, and the non-uniform chromium distribution, of the nickel-chromium matrix is substantially free of recrystallization and exhibits a grain structure and sub-grain structure having the same characteristics as the original nickel matrix. The product has high temperature tensile strength and stress-to-rupture life equal to or better than that of the starting material and also possesses excellent high temperature oxidation resistance.

The starting material for the process of the present invention can be any fully dense or substantially fully dense wrought dispersion strengthened nickel shape produced by any conventional or unconventional methods. However, since the high temperature strength characteristics of the starting material reflect directly in the high temperature strength characteristics of the final product, the preferred starting material is dispersion strengthened wrought nickel which itself has optimum high temperature properties.

Co-pending US. application Ser. No. 543,495, filed Apr. 18, 1966 describes a composite nickel-thoria powder which is particularly suitable for powder metallurgical fabrication of dispersion strengthened nickel, and copending US. application Ser. No. 465,291 (now Pat. No. 3,366,515) describes a preferred fabrication method whereby a microstructure which substantially improves strength properties can be developed in fully dense dispersion strengthened nickel. The invention is described hereinbelow as applied to starting material produced from composite nickel-dispersoid powder and by fabrication techniques described in the aforementioned pending applications. However, it is to be understood that the inventive process is applicable to any wrought shapes comprising a nickel matrix containing dispersed refractory particles and having any characteristic micro-structure.

Referring more specifically to the preferred embodiment, the composite nickel-refractory oxide powder of co-pending U .8. application Ser. No. 543,495 from which the preferred starting material is formed consists of irregular-shaped particles of nickel comprised of clusters of sub-particles of nickel between about 0.2 and about 0.5 micron in size. The sub-particles have ultra-fine thoria fixed in the surfaces thereof and may occur singly or be agglomerated in clusters up to 10 microns or more in size. The thoria particles preferably are between 10 and 30 millimicrons in size and are uniformly dispersed in the surfaces of the nickel sub-particles. Typical powders have an apparent density between 0.5 and 0.9 gram per cubic centimetre and Fisher subsieve number of less than 1.3 and preferably contains from about 2.0 to 4.0 percent by volume of one or more refractory oxides. The refractory oxide particles must have a melting point higher than the matrix metal, good thermal stability, low solubility in the matrix metal and should be nonreactive with the matrix metal at elevated temperatures of the order of 2400 F. There are a number of refractory oxides known to the art which satisfy the conditions necessary for use as the dispersed phase in dispersion strengthened nickel. For example, yttria, ceria and thoria have all been shown to be particularly suitable. Because of its ready commercial availability and high free energy of formation value, thoria is a preferred dispersoid. 1

The above-described nickel-dispersoid powders or any other powders of a similar nature can be processed into completely dense wrought dispersion strengthened shapes using any one of several techniques known to the art. One such technique involves a two-stage procedure whereby the powder is firstcompacted into a partially densified, self-supporting, green compact, such as by isostatic compaction to about 60% of absolute theoretical density, and the green compact is then hot worked to take a reduction in cross-sectional area of approximately 50% to produce a completely dense article. Alternatively, the powder or a preformed green compact or billet can be formed directly into dense finished or semi-finished shapes by extrusion.

In any case, the fully densified shape must normally be" subjected to further working operations to reduce it to final dimensions and/ or to produce a desirable microstructure in the matrix. As described in co-pending US. application Ser. No. 465,291 (now Pat. 3,366,515), a preferred working procedure comprises subjecting the densified material to a series of working cycles in which cold reductions in cross-sectional area of less than 20% are taken followed by a controlled anneal after each such reduction to relieve residual strains. The reductions in crosssectional area may be effected using any of the several known methods such as rolling, swaging or drawing. The intervening and-final anneals are carried out at a temperature which isbelow the melting point of the matrix. The annealing time and temperature is adjusted, having regard to the size and composition of the wrought shape to minimize recrystallization. For example, a 0.02 inch thick nickel-thoria strip containing 2.5% by weight thoria may satisfactorily be annealed by heating at 2200 F; for about 30 minutes. Annealing at excessively high temperatures for extended periods of time will result in recrystallization which is evidenced by the growth of large grains without a characteristic sub-grain structure. Recrystallization and loss of the grain and sub-grain structure results in a substantial reduction in high temperature strength properties.

By subjecting the wrought shape to repeated cold working cycles with cross-sectional area reductions of less than 20% and preferably less than 10% and controlled intervening anneals, a preferred grain structure is developed. Such a grain structure is described in co-pending US. application Ser. No. 465,291 (now Pat. No. 3,366,515). The grains are fibrous or lamellar and have awell developed substructure which is characterized by low angle sub-grain boundaries and an absence of recrystallization.

In. carrying out the present invention, the wrought dispersion strengthened nickel shape is alloyed with chromium by solid state diffusion of the chromium into the nickel matrix. A procedure which is particularly suitable for this purpose is chromizing. Chromizing is a surface treatment at elevated temperature generally carried out in pack, vapour or salt-bath in which an alloy is formed by the inward diffusion of chromium into the surface of the base metal. The known pack vapour deposition technique has been found particularly suitable for the chromium deposition step of this invention. This technique involves placing the article to be chromized in a container together with a mixture of chromium and alumina powders and heating the container contents. The powder mixture is composed of about 55% by weight chromium and 45% by weight alumina, with the alumina powder being provided to facilitate recovery of the article from the container at the termination of the heating period. The amount of chromium provided should be a substantial excess over the amount which it is desired to deposit onto the wrought dispersion strengthened nickel shape. An excess of chromium is helpful in ensuring a sufficient supply of chromium vapour at the surface of the wrought shape. Generally, it is desirable to deposit sufficient chromium to provide at least by weight chromium in the surface layer which is the minimum amount required to give some improvement in the oxidation resistance of the nickel. Preferably, suificient chromium should be deposited to provide -30% by weight chromium in the surface layer. The maximum amount of chrominum in the surface layer is about 35% by weight which is close to the solubility limit of chromium in nickel. The powder mixture containing the appropriate amount of chromium and alumina is placed within a steel container and the wrought shape is completely immersed within the powder mixture. The container is then sealed and heated together with its contents to a high temperature, for example, to about 2350 F. Heating is continued at the elevated temperature for a period of time sutficient to enable the amount of chromium desired in the final product to deposit on the wrought dispersion strengthened nickel shape. In order that the chromium content will be relatively uniform over the surface of the product, care must be taken to maintain a constant temperature throughout the article during the heating to avoid preferential deposition of the chromium at hot spots. Care should also be taken to protect the system from air contamination.

The temperature of the chromizing step is very important. The rate of chromizing depends upon the temperature used. The higher the temperature, the faster the vaporization and deposition of chromium. However, the chromizing temperature must be below the melting point of the matrix or the desired micro-structure will be lost and also agglomeration of the dispersed particles may occur. The normal temperature range from chromizing is from about 2000 F. to about 2400 F., preferably 2200 F. to 2400 F.

The length of time used for chromizing a particular wrought shape is determined by routine experiment. The temperature used, the thickness of the wrought shape, the amount of chromium available to vaporize and deposit and the amount of chromium desired in the final product will all influence the length of time which must be used.

The product is characterized by a chromium concentration gradient which decreases from the surface inwardly. As an example, a 1 in. x 6 in. x 0.020 in. chromized nickel-thoria strip chromized according to the invention had a chromium content of 32.7% at the surface and 9.0% at the centre.

The matrix of the product contains nickel and chromium, although now in uniform concentration across its section, and exhibits essentially the same grain and subgrain structure as that of the nickel matrix of the starting material. That is, surprisingly, the prolonged heating during chromium deposition the massive infusion of chromium into the nickel with accompanying volume increase of the matrix does not alter the basic characteristics of grain and sub-grain structure which was present in the nickel matrix of the starting material. No agglomeration of the dispersoid occurs when chromium deposition is carried out under the controlled conditions described hereinabove. Thus, in the preferred embodiment of the invention where the nickel matrix of the starting material contains highly disseminated refractory particles and has a well developed fibrous micro-structure with a polygonized sub-structure, the final nickel-chromium alloy product exhibits the same characteristics. The product retains the high temperature strength properties attributable to these characteristics and, at the same time, has the superior high temperature oxidation resistance of nickel-chromium alloys. 1

It is anticipated that other alloy metals, such as iron, cobalt, tungsten, molybdenum, niobium and tantalum may be included within the composition of the final product. Metal powders can be mixed with the nickel and dispersoid prior to processing or they can be introduced later by techniques such as vapour-deposition.

The following example illustrates a practical embodiment of the invention.

EXAMPLE (A) Preparation of wrought dispersion strengthened nickel starting material (prior art) Nickel-thoria powder was compacted in a 1.25 inch by 2.4 inch die under a ton load to make a 60 gram billet 0.20 inch in thickness. The nickel-thoria powder was obtained by the hydrometallurgical process described in co-pending US. application Ser. No. 543,495 and had the following characteristics.

Analysis:

' Thoria percent by weight 3.1 Sulphur percent by weight 0.002 Carbon percent by weight 0.007 H loss percent" 0.8 Nickel and incidental impurities Balance Apparent density gram/cc 0.99 Fisher subsieve No. 0.53 Thoria size range millimicrons 10 to 30 The billet which was about 60% fully dense, was heated in a flowing hydrogen atmosphere to a temperature of 2200 F. to prepare it for hot working and to remove any nickel oxide present. It was then hot rolled to take a 50 percent reduction, resulting in a substantially 100% dense strip product 0.10 inch in thickness. The strip was subsequently annealed in a flowing hydrogen atmosphere at 2200 F. for 30 minutes.

The fully dense strip was cooled and cold rolled to take a 10% reduction. Following cold working, the strip was annealed at 2200 F. for 30 minutes and the cold rollanneal working cycle was repeated with 10% reductions until the strip had undergone a total of 15 work-anneal cycles resulting in a final strip thickness of 0.02 inch.

The ultimate tensile strength of the worked strip at 2100 F. was 13,800 p.s.i.

(B) Preparation of dispersion strengthened nickel-chromium alloy product in accordance with the invention 55 grams of -200 +325 Tyler mesh screen, pure commercial grade chromium powder was mixed with 45 grams of alumina powder in a cone blender for 30 minutes. The powder mixture was placed in a 5 in. x 6 /2 in. x l in. ID. stainless steel furnace boat. A piece of nickelthoria strip 1 in. x 6 in. x 0.02 in. dimension from part A above was completely immersed within the powder mixture in the boat. A stainless steel plate was welded on the boat and the boat and contents were then placed in a furnace and heated to 2200 F.

The boat and contents were maintained at this temperature for hours. On cooling, it was found that 20% by weight chromium had deposited on the strip. The chromium gradient within the strip varied from 32.7% at its surface to 9.0% at its centre.

It will be understood, of course, that modifications can be made in the preferred embodiment of the present invention as described hereinabove without departing from the scope and purview of the appended claims.

What we claim as new and desire to protect by Letters Patent of the United States is:

l. A dispersion strengthened nickel-chromium alloy product which comprises a sheet formed of wrought nickel containing a plurality of sub-micron dispersoid particles uniformly disseminated therethrough and having a characteristic microstructure developed by its previous processing history, said strip having chromium diffused therein such that the surface layer thereof contains from about 10 to about 35% by weight chromium and the chromium content decreases inwardly towards the centre of the sheet section.

2. A product according to claim 1 in which the surface layer contains about 20% by weight chromium.

3. A product according to claim 2, in which the microstructure of said nickel strip is characterized by fibrous grains having a polygonized substructure.

References Cited UNITED STATES PATENTS L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner US. Cl. XR.

UNITED STATES PATENT OFFICE' CERTIFICATE OF CORRECTION patent 596 Datd September 19, 1972 Invent Robert Wi'l I iam Fraser et a1 It is' certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' z Column 1, in the heading lines 8, 9, l0 and 11 shoul read No, Drawing, a continuationin-part of application Serial No. 815, 780 filed April 14, 1969 and now abandoned, which, in turn, is a divisional of Serial No 570, 389, filed July 22,

1966, now U. S Patent No. 5,454 ,431

Signed and sealed this 1st day of May 1973.

(SEAL) Attest:

EDWARD M.PLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents I saw 1050 59) USCOMM-DC 60378-P69 UIS, GOVERNMENT PRlNTlNG OFFICE I Q59 0"35533fl,

Patent September 19, 1972 3,692,596 Datd Inven Robert will iam Fraser et a1 It is' certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, in the heading lines 8, 9, 10 and 11 shoul read No Drawing, a continuation-in-part of application Serial No. 815,780, filed April 14,- 1969 and now abandoned, which, in turn, is a divisional of Serial No. 570,389, filed July 22,

1966, now U. 5. Patent No. 3,454,431

Signed and sealed this 1st day of May 1973.

(SEAL) Attest:

ROBERT GOTTSCHALK EDWARD M.PLETCHER,JR.

Commissioner of Patents Attesting Officer USCOMM-DC 60376-P69 w IIIS. GOVERNMENT PRINTING OFFICE: I959 0-366-334,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pat nt N Dated September 19,

Inventofls) Rnhert Wil I iam Fraser 61: a1

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, in the heading lines 8, 9, l0 and 11 shoul read No Drawing, a continuation-in-part of application Serial No. 815,780, filed April 14, 1969 and now abandoned, which,

in turn, is a divisional of Serial No. 570,389, filed July 22, 1966, now U. S. Patent No. 3,454,431

Signed and sealed this 1st day of May 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents av F O-1050 (10-69) USCOMM oc 60376 P69 LLS. GOVERNMENT PRINTING OFFICE: I989 0-366-38L 

